High-pressure hydraulic valve

ABSTRACT

The present invention relates to a high-pressure hydraulic valve in which the housing ( 1 ) at least and also preferably the operational parts, such as the main control piston ( 3 ), the manometer piston ( 6 ) and the like which are arranged inside said housing ( 1 ) and influence the hydraulic oil flow, are made of an aluminium alloy that can be hardened and tempered. The weight of the high-pressure hydraulic valve can therefore be substantially reduced. The surfaces of the parts made of the aluminium alloy that can be hardened and tempered are advantageously submitted to anodic oxidation in order to increase even more their wear resistance. This type of high-pressure hydraulic valve with a particularly light weight is mainly intended for use in mobile hydraulic systems such as those used in mobile cranes, in lifting platforms and in excavators.

The invention relates to a high-pressure hydraulic system having a valvefor varying the flow of hydraulic oil between a pump and a hydraulicload.

Such high-pressure hydraulic valves, in various types and constructions,are employed in mobile hydraulic systems. Examples of such differenttypes are proportional valves, load holding brake valves, andblocking/safety valves. The mobile hydraulic systems are components ofdredgers, cranes and other lifting devices, for instance. In suchapplications, the weight of the hydraulic units plays a role. The weightshould be as low as possible.

Typically, the housings of high-pressure hydraulic valves are made fromgray cast iron or spherulitic graphite iron, and the internal parts,some of them movable, are made from hardened and tempered steel. Theweight of the high-pressure hydraulic valve is determined substantiallyby the weight of this housing. One such high-pressure hydraulic valve isknown from WO92/07208. In U.S. Pat. No. 5,556,075 that matured from thisapplication, it is mentioned as an essential effect of the inventionthat the weight is quite low. Thus in both of these previously knownapplications it is proposed that a part of the valve spindle, or theentire valve spindle and valve cone assembly, be produced from a ductilematerial, such as copper, brass, or in particular aluminum. This isintended to improve the resistance to leakage. In the case wherealuminum is used, a reduction in weight is indeed achieved, but in viewof a housing whose mass predominates, this reduction is only minimal. Aweight reduction for the valve spindle and valve cone assembly alsoleads to a lower inertial mass and thus to a lesser dynamic pressureloss in periodic switching. Because of the circumstance alreadymentioned, that the weight of the valve spindle and valve cone assemblyis only a small proportion of the total weight, the saving in weight forthe high-pressure hydraulic valve intrinsically is not very significant.Furthermore, there is the problem of the different coefficients ofthermal expansion of the various materials.

The goal of making machine components as light in weight as possible hasexisted for a long time. Hence lightweight structural materials havelong been employed in a great many fields of industry. As an example, itcan be mentioned that such lightweight materials as titanium andaluminum are used in passenger-carrying motor vehicles as well. This isknown for instance from U.S. Pat. No. 5,169,460. Since as noted, therehas long been a need to reduce weight, it can be concluded that inindustrial fields in which such lightweight materials have not yet beenused, strong arguments exist that preclude the use of such materials.

The object of the invention is to reduce the weight of a high-pressurehydraulic valve markedly further, without adversely changing itsutility. Advantageous refinements will become apparent from thedependent claims.

The total weight of a high-pressure hydraulic valve is definitivelydetermined by its housing. Such shape-related data as size and wallthickness, on the one hand, and the specific weight of the material usedon the other, play a significant role. The size of the housing isinfluenced for instance by the size of the flow through it that is to becontrolled, as well as by the structural design that is associated withthe mode of operation. The maximum pressure that the high-pressurehydraulic valve has to control also has a determining influence on thehousing size, for instance on the minimum thickness of walls. Hydraulicsystems that can be classified as high-pressure systems arecharacterized by operating pressures of more than 100 bar. Systems witha rated operating pressure of 420 bar are known. In this respect itshould be noted that precisely in such mobile systems as cranes anddredges, peak loads can certainly occur that exceed the rated operatingpressure. Although as a rule safety devices that are intended to preventgreater loads are present, nevertheless even in such cases, dynamic peakloads can occur. The manufacturer of high-pressure hydraulic valves musttherefore provide proof that the function and durability of the unit isassured even at pressures that are a multiple of the rated operatingpressure, for instance four times the rated operating pressure.

In contrast to the time-tested prior art, in which the gray cast iron orpreferably spherulitic graphite iron GGG 40 is used as the housingmaterial and hardened steel is used for the internal parts, it isproposed according to the invention that a precipitation-hardenablewrought aluminum alloy, such as AlZnMgCu, be used as the housingmaterial. Since the specific weight of such an alloy is approximately2.8 g/cm³, while that of spherulitic graphite iron GGG 40 has a value of7.25 g/cm³, a saving in weight for the weight of the housing, assumingthe same dimensions, of about 60% is attainable.

Often, housing parts for high-pressure hydraulic valves are made notfrom molded castings but rather from solid blocks. On the one hand thisis because the numbers of pieces are so low that producing molds andmold cores for the internal hollow spaces is unfeasible because of thehigh cost, and on the other because often a plurality of parts arecombined as elements that can be lined up to make larger units. Herethere are advantages if the housings have a blocklike shape. In suchblocklike housings, the absolute weight advantage, expressed inkilograms per component, is especially significant when aprecipitation-hardenable aluminum alloy is used instead of spheruliticgraphite iron GGG 40.

The mechanical specifications for the above materials, such as tensilestrength, Brinell hardness, modulus of elasticity, and tensile yieldstrength, have long been known. Although the tensile strength and thetensile yield strength of precipitation-hardenable aluminum alloys arefor instance higher than the corresponding values for spheruliticgraphite iron GGG 40, the profession has long shied away from usingprecipitation-hardenable aluminum alloys as material for the housings ofhigh-pressure hydraulic valves.

What evidently also stood in the way of the solution according to theinvention was that the wear resistance of spherulitic graphite iron GGG40 is considered to be extraordinarily high, while aluminum alloys ingeneral are not considered especially abrasionproof. However, as testshave shown, in hydraulic equipment this aspect lacks overridingsignificance, because the hydraulic oil present in such equipment actsas a lubricant in the event of sliding stress. As an advantageousfeature of the invention, however, a surface treatment of the componentsmade from the aluminum alloy can be done, preferably an anodicoxidation. Such methods have already long been known per se. Oneadvantageous method for the intended purpose is the creation, by themethod known as HART-COAT® (trademark of the corporation doing businessas AHC-Oberflächentechnik, Friebe & Reininghaus GmbH), of a coating witha comparatively very slight pore volume. In this method, the anodicoxidation takes place in a specially composed acid electrolyte, which iscooled during the process.

An exemplary embodiment of the invention will be described in furtherdetail below in conjunction with the drawing.

FIG. 1 shows a schematic fragmentary section through a high-pressurehydraulic valve.

FIG. 2 shows in diagrammatic form a hydraulic system which includes thehigh-pressure hydraulic valve.

Reference numeral 1 represents a housing, which is block- shaped.According to the invention, this housing 1 comprises aprecipitation-hardenable wrought aluminum alloy, for instance of thetype AlZnMgCu. In the interior of the housing 1, there are hollowspaces, in which functional components are located. The hollow spacesare machined out of the block, for instance by drilling and/or milling.Thus the high-pressure hydraulic valve has two connections 2, one ofwhich communicates, via a hydraulic line 10 (FIG. 2), with the hydraulicload 11, such as a hydraulic actuator. The other connection leads viathe same kind of hydraulic line 12 to a pump 13, also depicted in FIG.2.

The high-pressure hydraulic valve shown in the drawing is a module,which along with a proportional multiposition valve and a two-waypressure compensator also contains other elements, but these are notshown here because they are not essential to the invention. Examples ofsuch other elements are adjusting devices for maximum pressure, maximumstroke, and pressure difference.

As functional components that are disposed inside the housing 1 and thatvary the flow of hydraulic oil, the following can be seen in thedrawing: a main control spool 3, which is disposed displaceably alongits axis 4 in a first chamber 5, and a pressure compensator spool 6,which is disposed displaceably along its axis 7 in a further chamber 8.The pressure compensator spool 6, together with part of the wall of thechamber 8, can form a check valve 9. The above elements determine themode of operation of the high-pressure hydraulic valve. However, theywill not be described further here, because their mode of operation isnot essential to the invention.

In high-pressure hydraulic valves of this kind in the prior art, thefunctional parts, such as the main control spool 3 and the pressurecompensator spool 6, are made of hardened steel. The hardening is doneat the conclusion of the production process. However, since dimensionalchanges known as warping occur in the heat treatment, and especially inparts machined by metal cutting, remachining is typically necessary inorder to achieve the required dimensional precision of the parts for thesake of the function.

If the functional parts that influence the flow of hydraulic oil, suchas the main control spool 3 and the pressure compensator spool 6,comprise hardened steel, while the housing 1 comprises a wroughtaluminum alloy, then for designing the construction and the dimensionsand tolerances of the parts, it must be taken into account that thecoefficients of thermal expansion of steel and aluminum alloys aremarkedly different, namely about 12×10⁻⁶/° C. for steel, andapproximately twice as high, namely 23 to 24×10⁻⁶/° C. for aluminumalloys. In hydraulic systems that are stationary when in operation, thisdifference does not play a significant role, for instance if the systemis set up in a factory space. In mobile systems that have hydraulicdevices, such as cranes and dredgers, however, because these devices areused in the open quite considerable temperature differences occurbecause of changing meteorological conditions. Accordingly, the incidentmaximum temperature values have to be considered in designing the parts.This is feasible per se, especially since the temperature also acts onthe properties of the hydraulic oil, so that the engineer designing thehigh-pressure hydraulic valve has to take temperature factors intoaccount anyway.

However, it is advantageous if the functional parts, such as the maincontrol spool 3 and pressure compensator spool 6, are also of aprecipitation-hardenable wrought aluminum alloy, such as AlZnMgCu. Thusat temperatures that depart from the normal, no additional problems withtolerances that would have to be taken into account occur.

It is advantageous if the blank, such as a blocklike body, that existsbefore the machining is done to form the housing 1, is already in theprecipitation-hardened state. The machining to form the finished housingis accordingly done in the already precipitation-hardened state. Thishas a number of advantages: First, the manufacturer of the parts doesnot need any heat treatment system suitable for theprecipitation-hardening of the material. On the other hand, as a resultof the precipitation-hardening, dimensional changes in the housing 1machined to final form no longer occur. However, a highly essentialadvantage results from the fact that the metal-cutting machining is madeeasier. Aluminum materials are generally known to be ductile and tough.This requires specially ground metal-cutting tools and nevertheless veryoften leads to the creation of so- called built-up edges, which alsoadversely affects the dimensional stability. It has been found that thecutting conditions are substantially more favorable when the machiningis done in the precipitation-hardened state. Thus work can be done withcutting speeds that are even higher than those in gray cast iron.

What has been said about the production of the housing 1 applies equallyto the functional parts to be made from a precipitation-hardenablewrought aluminum alloy, such as the main control spool 3 and pressurecompensator spool 6.

Following the production of the parts, that is, the housings 1, maincontrol spool 3, pressure compensator spool 6, and the like, the anodicoxidation of the parts will advantageously be done. It should be takeninto account that the buildup of the film during the anodic oxidationcauses a dimensional change. Starting at the surface, aluminum atoms areconverted into aluminum oxide molecules. The film thus increases indepth on the one hand but on the other also leads to an increase insize, since the aluminum oxide formed in the anodic oxidation takes upmore space. The dimensional changes are controllable, however, becausethey are associated strictly with the thickness of the film formed. Itis accordingly possible to manufacture the crude parts with a certainundersize, from which by the creation of an oxide film, given a certainthickness of the oxide film, a finished part that is preciselydimensionally stable is created.

The oxide film created anodically advantageously has a thickness ofabout 200 μm.

Because the aluminum oxide film has the property of being a very poorelectrical conductor, in a mixed construction with aluminum and steel,the additional advantage is also attained that contact corrosion betweenmetal components of different material with a different position in theelectrical voltage series is maximally precluded. This is significanteven in the advantageous case, since both the housing 1 and thefunctional parts, such as the main control spool 3, pressure compensatorspool 6 and the like, are made from a wrought aluminum alloy, becausesprings, for instance, that act on such functional parts and/or arebraced on the housing 1 are typically made of steel.

In the housing 1, the film created by the anodic oxidationsimultaneously meets multiple demands. On the surface parts located onthe outside, the film created functions both to protect againstcorrosion and to provide an aesthetically attractive appearance. Evenpainting the surface, which is usually done, can accordingly even bedispensed with under some circumstances. However, if painting is stilldesired, then the anodically created oxide film offers an excellent basefor adhesion, so that the adhesion base preparation and priming, whichas a rule are necessary in workpieces of aluminum materials and areespecially complicated, can be omitted.

On the surfaces of the internal hollow spaces and chambers such aschamber 5 for the main control spool 3 and chamber 8 for the pressurecompensator spool 6, not only corrosion protection but above all wearprotection is of prime importance. It has been found that high-pressurehydraulic valves of the type described above, having the characteristicsof the invention and the films created by anodic oxidation and found tobe advantageous, are extraordinarily wear-resistant. They are notinferior to conventional valves in terms of their service life.

It has been found, especially strikingly, that the hysteresis of ahigh-pressure hydraulic valve of this kind is less than in conventionalvalves. This reduced hysteresis has a very advantageous effect in termsof the precision of positioning in hydraulic adjusting devices as wellas with regard to acceleration and deceleration operations. In otherwords, the dynamic behavior is markedly improved.

The exemplary embodiment described above pertains one specificembodiment of a high-pressure hydraulic valve. In other designs ofhigh-pressure hydraulic valves, the functional parts are sometimescalled by other names. The functional parts are accordingly understoodto include these other kinds of parts as well, such as valve bodies,valve cones, open- and closed-loop control spools, and so forth.

Without departing from the scope of the invention, the housing 1 canalso be made from a precipitation-hardenable cast aluminum alloy, withthe production of a blank for the housing 1 being done by casting. Thegeneral teaching of the invention is accordingly that the housing 1 ismade from a precipitation-hardenable aluminum alloy. The anodicoxidation can advantageously also be done in the same way when a castaluminum alloy is used.

What is claimed is:
 1. In a high pressure hydraulic system operable at hydraulic fluid pressures above 100 bar and comprising a pump for providing pressurized hydraulic fluid to a hydraulic load, and a high pressure hydraulic valve for varying the flow of hydraulic fluid between said pump and said hydraulic load, the improvement which comprises a valve housing of precipitation hardened aluminum alloy having at least one chamber therein for receiving a movable functional part for affecting the flow of the hydraulic fluid.
 2. The combination of claim 1, wherein said precipitation hardened aluminum alloy is an AlZnMgCu alloy.
 3. The combination of claim 1 or 2, wherein said valve housing has anodically oxidized surfaces to protect against wear and corrosion.
 4. The combination of claim 1, wherein surfaces of said aluminum alloy valve housing comprise an approximately 200 μm thick anodically created oxide film.
 5. The combination of claim 1, wherein said functional part comprises a precipitation hardened aluminum alloy.
 6. The combination if claim 5, wherein said functional part is a valve spool, and surfaces of said valve housing and said spool comprise an anodically created aluminum oxide film.
 7. The combination of claim 1, wherein said functional part comprises an AlZnMgCu alloy.
 8. A method for producing a hydraulic valve according to claim 1, said method comprising the steps of precipitation hardening the valve housing by heat treatment, and then shaping the housing by at least one metal-cutting process.
 9. A method according to claim 8, comprising the additional step of anodically oxidizing surfaces of said housing after said metal cutting.
 10. A method according to claim 8, wherein a blank for said housing is cast from a precipitation-hardenable aluminum alloy.
 11. A method according to claim 8, wherein a blank for said housing is wrought from a precipitation-hardenable aluminum alloy.
 12. A method according to claim 11, wherein said alloy is an AlZnMgCu alloy.
 13. A method according to claim 8, additionally including comprising forming said movable functional part of said valve from a precipitation-hardenable wrought aluminum alloy.
 14. A method according to claim 13, wherein said movable functional part is formed of an AlZnMgCu alloy.
 15. A method according to claim 13, additionally comprising anodically oxidizing surfaces of said functional part. 